1. Field of the Invention
The present invention relates to an electrode catalyst for fuel cells and a process for producing the same.
2. Description of the Related Art
Fuel cells convert chemical energy directly into electric energy by using electrochemical reactions of gasses. Since fuel cells are not subjected to the limitation of the Carnot efficiency, it exhibits high power generation efficiency. Moreover, since fuel cells emit clean exhaust gases, they adversely affect environments extremely less. Accordingly, fuel cells have been recently expected to be used in a variety of applications such as power generation applications, batteries for less-polluting automobiles. Fuel cells can be classified by their electrolytes. For example, the following fuel cells have been known: phosphoric acid-type fuel cells, molten carbonate-type fuel cells, solid oxide-type fuel cells, solid polymer-type fuel cells and the like.
In general, in fuel cells, an electrode-electrolyte assembly makes a power generation unit in which a pair of electrodes are disposed on the opposite sides of an electrolyte. The electrodes are a fuel electrode and an air electrode, respectively. A fuel gas, such as hydrogen and hydrocarbons, is supplied to the fuel cell, and oxygen or air is supplied to the air electrode, respectively. Electricity is collected from fuel cells by developing electrochemical reactions at the 3-phase boundary between the gases, the electrolyte and the electrodes.
Electrode catalysts are used at the respective electrodes, the fuel electrode and the air electrode, in order to develop the aforementioned reactions. As for the electrode catalysts, the electrode catalysts in which platinum is loaded on conductive supports such as carbon have been the mainstream. However, since platinum is expensive, trials have been carried out variously to enhance the catalytic activities by alloying platinum with less expensive metals. For instance, in Japanese Unexamined Patent Publication (KOKAI) No. 4-371,230, a platinum-cobalt 2-way alloy electrode catalyst is disclosed. Moreover, in Japanese Unexamined Patent Publication (KOKAI) No. 6-176,766, a platinum-nickel-cobalt 3-way alloy electrode catalyst is disclosed.
However, according to the follow-up experiments conducted by the present inventors, the catalytic activities of the aforementioned alloy electrode catalysts were not satisfactory. The present inventors further studied on the alloy electrode catalysts. As a result, it was found that the base metals, such as cobalt and nickel, were not solved in platinum entirely but were partially turned into metallic oxides and the like to remain in alloy. Further, since most of the metallic oxides existed so as to cover the surface of the alloy particles, the metallic oxides reduced portions of the alloys which made a reaction field. Thus, it is believed that the metallic oxides lower the usable ratio of the alloys as an electrode catalyst. Furthermore, in a case where such alloys were used as a catalyst for an air electrode of solid polymer-type fuel cells, since the reaction field was a strongly acidic atmosphere, the base metals which formed the metallic oxides eluted out of the metallic oxides as metallic ions. The eluting metallic ions were ion-exchanged with the hydrogen ions in the electrolytes to lower the ionic conductivities of electrolytes. Thus, the metallic oxides degraded the battery performance of the resulting fuel cells. Moreover, due to the presence of the metallic oxides, it was difficult to uniformly form a catalytic layer on the surface of electrolytes.